As a blow molding process for the production of a hollow body of thermoplastic material by expansion of a preform in a blow molding mold is subject to limitations in regard to the shape that can be imparted to the hollow body upon expansion of the preform in the blow molding mold to give the expanded hollow body, it has long been the practice for products which are of a complicated shape and configuration to be produced by a blow molding mold by means of a procedure wherein at least one separately produced additional portion is introduced into the blow molding mold before the latter is closed around the preform, whereupon in the subsequent expansion operation a connection is formed between the preform, or the hollow body which is formed therefrom in the course of expansion, and the additional portion in the mold, in such a way that parts of the surface of the additional portion are satisfactorily joined to a portion of the surface of the preform or the hollow body produced therefrom. In the majority of cases the preform and additional portion consist of the same plastic material so that, as the preform is in a hot-plastic condition in the expansion operation and is thus at a temperature which permits a welded joint to be formed between the preform and the additional portion, the additional portion can be secured to the hollow body by a welded joint in that way. In that respect, generally depending on the plastic material constituting the preform and the wall thickness thereof, a part of the thermal content of the preform is also used to raise the additional portion to a temperature which permits a good welded joint to be formed. In many situations in that respect the procedure adopted is such that the preform is formed at a temperature which is higher than that which would be required for the subsequent expansion operation in the blow molding mold and for the formation of welded seams to close the preform, in order in that way additionally to make available heat which, when contact occurs between the expanded preform and the additional portion, raises the latter at its contact surface to a temperature which is sufficiently high also to cause plasticization of the additional portion in the marginal layer which bears the contact surfaces. That mode of operation normally readily permits the attachment of prefabricated additional portions of that kind, for example consisting of polyethylene, to blow-molded bodies which also consist of polyethylene.
It is also possible for the additional portion to be preheated if the thermal content of the preform is not sufficient to bring about a satisfactory welding effect. In situations in which however preheating results in plasticization of the surfaces to be welded, handling of the additional portion when it is introduced into a suitable seat in which it is accommodated in the blow molding mold is made correspondingly difficult.
By way of indication, a hollow body with additional portions joined thereto is disclosed in German utility model No 1 852 986.
In the course of expansion of the use of the blow molding process, which has occurred in recent years, use is increasingly made of plastic materials which are more difficult to process, more specifically insofar as adding an additional portion to a hollow body in the above-described simple manner does not always result in the formation of welded joints of adequate strength. That can be attributed on the one hand to the fact that such plastic materials which for example may be thermoplastic elastomers require much closer temperature limits in the plasticization operation, that is to say when they are being processed in an extruder, so that there is no possibility of adopting a higher temperature for the preform in order in that way to provide the heat required to produce a sufficiently strong welded joint. Another problem which can arise in the case of plastic materials of that kind is that they have a tendency to suffer oxidation at their surfaces in the hot-plastic condition, thereby resulting in the formation at the surface of the material of a layer which makes it difficult or even impossible to provide a welded joint of adequate strength, even under conditions which in themselves are advantageous in respect of temperature, pressure and time. Admittedly, unless operation is carried out in an inert atmosphere, a certain degree of oxidation for example of the surfaces of a preform due to oxygen in the air can scarcely be avoided; however, as the degree of oxidation essentially depends on the time involved, the formation of an oxidation layer on the surfaces of the preform, such as to counteract the formation of a welded joint, can be avoided by carrying out the process in a suitable fashion, more specifically by producing the preform quickly. However, that is not always possible when the additional portion is to be preheated, unless the preheating operation is also effected in an inert atmosphere. In that situation, the difficulties already mentioned above in regard to handling an additional portion which has been preheated for plasticization thereof are also encountered. On the other hand however preheating of that kind will be necessary in those situations in which the amount of heat required to produce a welded joint can no longer be provided by the preform alone, in contrast to the above-described operating procedures. That will be the case in particular when the material forming the preform requires closer temperature limits to be observed, as already mentioned above.